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Costine A.,CSIRO | Nikoloski A.N.,Murdoch University | Costa M.D.,CSIRO | Chong K.F.,Murdoch University | And 2 more authors.
Minerals Engineering | Year: 2013

Brannerite is a refractory uranium mineral from which it is very difficult to liberate the uranium. Hence in commercial mineral processing operations, brannerite often reports to the residue. This paper will show that for a pure form of natural brannerite nearly complete extraction of uranium (∼99%) is achievable under practical conditions. The efficient extraction of uranium from ores containing brannerite requires a detailed understanding of the fundamental mechanisms governing the rate and extent of dissolution. These mechanisms are often complicated by the presence of gangue minerals which consume reagents and impact on the solution chemistry. In this study, the acidic ferric sulphate leaching of an exceptionally pure, natural brannerite mineral (35.8% U, 20.1% Ti) was investigated under atmospheric conditions. Hence the variation in mineral composition was not present as a complicating factor and the results were able to identify some of the inhibiting mechanisms, and also the preferred conditions for the leaching of brannerite in an acidic ferric sulphate system. The effects of temperature (40-80 C), ferric ion concentration (0-100 g/L), H2SO4 concentration (10-200 g/L), redox potential (424-752 mV vs. Ag/AgCl), and particle size on uranium and titanium extractions were studied for leach times up to 48 h. Under relatively mild conditions (40 C, 24 h leach time, 40 g/L H2SO4), the extent of uranium extraction was 94.4%. The extractions improved with the use of a higher temperature, a finer particle size, and a longer leach time. The presence of ferric iron was essential for enhanced dissolution rates, but had only a minor effect on the final uranium extractions, particularly at 60 C and 80 C. All of the leach residues studied had some crystalline anatase (TiO2) and lead sulphate (anglesite) present. A strong correlation was found between the concentrations of unleached uranium and the amount of titanium precipitated in the residues, which could be explained by the observation of a Ti-enriched diffusion layer on the surface of the dissolving grains of brannerite, which hindered the extraction process. These findings further the current understanding of the extraction process and lead a step closer to elucidation of the mechanism of the extraction process. © 2013 Elsevier B.V. All rights reserved. Source

Powell K.J.,University of Canterbury | Brown P.L.,Research Avenue | Byrne R.H.,University of South Florida | Gajda T.,University of Szeged | And 4 more authors.
Pure and Applied Chemistry | Year: 2013

The numerical modeling of ZnII speciation amongst the environmental inorganic ligands Cl-, OH-, CO32-, SO42-, and PO43-requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log10βp,q,r° valid at Im = 0 mol·kg-1 and 25 °C (298.15 K), and reports the empirical reaction ion interaction coefficients, Δ∈required to calculate log10βp,q,r values at higher ionic strengths using the Brønsted-Guggenheim-Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ΔrH, are reported where available. There is scope for additional high-quality measurements for the Zn2+ + H+ + CO32-system and for the Zn2+ + OH-and Zn2+ + SO42-systems at I > 0. In acidic and weakly alkaline fresh water systems (pH <8), in the absence of organic ligands (e.g., humic substances), ZnII speciation is dominated by Zn2+(aq). In this respect, ZnII contrasts with CuII and PbII (the subjects of earlier reviews in this series) for which carbonato-and hydroxido-complex formation become important at pH > 7. The speciation of ZnII is dominated by ZnCO3(aq) only at pH > 8.4. In seawater systems, the speciation at pH = 8.2 is dominated by Zn2+(aq) with ZnCl+, Zn(Cl)2(aq), ZnCO3(aq), and ZnSO4(aq) as minor species. This behaviour contrasts with that for CuII and PbII for which at the pH of seawater in equilibrium with the atmosphere at 25 °C (log10 {[H+]/c°} ≈ 8.2) the MCO3(aq) complex dominates over the MCln(2-n)+ species. The lower stability of the different complexes of ZnII compared with those of CuII, PbII, and CdII is also illustrated by the percentage of uncomplexed M2+ in seawater, which is ca. 55, 3, 2, and 3.3% of [MII]T, respectively. © 2013 IUPAC. Source

Trenfield M.A.,Environmental Research Institute of the Supervising Scientist | Trenfield M.A.,University of Queensland | McDonald S.,Curtin University Australia | Kovacs K.,University of Szeged | And 7 more authors.
Environmental Science and Technology | Year: 2011

Fulvic acid (FA) from a tropical Australian billabong (lagoon) was isolated with XAD-8 resin and characterized using size exclusion chromatography, solid state cross-polarization magic angle spinning, 13C nuclear magnetic resonance spectroscopy, elemental analysis, and potentiometric acid-base titration. Physicochemical characteristics of the billabong FA were comparable with those of the Suwannee River Fulvic Acid (SRFA) standard. The greater negative charge density of the billabong FA suggested it contained protons that were more weakly bound than those of SRFA, with the potential for billabong water to complex less metal contaminants, such as uranium (U). This may subsequently influence the toxicity of metal contaminants to resident freshwater organisms. The complexation of U with dissolved organic carbon (DOC) (10 mg L-1) in billabong water was calculated using the HARPHRQ geochemical speciation model and also measured using flow field-flow fractionation combined with inductively coupled plasma mass-spectroscopy. Agreement between both methods was very good (within 4% as U-DOC). The results suggest that in billabong water at pH 6.0, containing an average DOC of 10 mg L-1 and a U concentration of 90 μg L-1, around 10% of U is complexed with DOC. © 2011 American Chemical Society. Source

Cao G.,Level Inc | Zhang X.,Research Avenue | Zhang H.,Level Inc
TMS Light Metals | Year: 2014

The importance of the cathode assembly thermal-electrical and thermal-mechanical performance cannot be overstated when designing an aluminum reduction cell. However, it is extremely difficult to measure in-service cathode assembly performance or to infer in-service behaviour from any measurements of cathode assemblies at room temperature. A complete thermo-electrical and thermo-mechanical modelling approach has been developed to conduct sequentially coupled simulation of the cathode assembly lifecycle performance. The modelling starts with the cathode rodding process which allows the air gap between the cast iron and carbon to be predicted. The results are built into the subsequent thermo-electrical and thermo-mechanical models of the in cell operation. The cathode voltage drop is then estimated by coupling the predicted contact pressure and temperature with the electrical contact resistance. The model predicted air gaps as well as cathode voltage drop savings due to design changes have been validated by carefully designed experimental measurements for various cathode assembly designs. Copyright © 2014 by The Minerals, Metals & Materials Society. Source

Bhargava S.K.,RMIT University | Ram R.,RMIT University | Pownceby M.,CSIRO | Grocott S.,Research Avenue | And 3 more authors.
Hydrometallurgy | Year: 2015

Uraninite is mined/processed more than any other uranium mineral for the production of uranium based compounds that are subsequently used to produce nuclear fuel. This review article provides a concise account of the available literature on one of the major processes involved in processing uraninite bearing ores, acid leaching. Improvements in the processes used to leach uraninite are required in order to ensure efficiency in the processing of lower grade uraninite bearing ores with minimal environmental impacts. This in turn requires improvements in our understanding of uraninite leaching. The main topics covered in this review include: uraninite structure, composition and low temperature geochemistry; the chemistry of uraninite leaching; key factors that influence uraninite leaching; and leach process technologies. The research that has been reviewed clearly establishes the influence of parameters such as temperature, acid concentration and particle size. The influence of other parameters however, such as solution Fe3 + to Fe2 + ratio (solution Eh), total Fe concentration, foreign ions present in the leach slurry and uraninite composition is yet to be established. Based on the literature available on the aforementioned factors the chemistry/processes involved in uraninite leaching are quite complex and require significant further studies. From the literature reviewed it is clear that variations in mineral chemistry in individual ore types across multiple deposits also make it essential that before any extraction process is considered, detailed ore characterisation studies of pre- and post-leach residues are of vital importance in order to fully understand the interrelationship between chemistry, mineralogy (ore and gangue), mineral liberation and potential leaching behaviour of uranium. © 2014 Elsevier B.V. Source

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